If you look at ?print, the generic function, you'll see some classes have the digits= argument, which is really all you need, so you could guess to do what I originally suggested (as I did), but then since we see it's of class htest, we can find out about it via getAnywhere:

getAnywhere(print.htest)

and seeing it's in stats (as would have been the obvious first guess anyway), we could also have seen the code by:

stats:::print.htest

So either of those last two show you exactly what it does, and also how to modify its behavior using print, because you can see it definitely has the 'digits' argument. You could even write your own function if you really had a mind to.